1. Field of the Invention
This invention relates to the bonding of cellular glass bodies and more particularly to a bonded cellular glass body system and mortar therefor wherein the mortar includes a substantial portion of a glass powder having a composition substantially the same as that of the cellular glass being bonded.
2. Description of the Prior Art
Acid-resistant mortars and coating for insulating systems are well known in the art. Such mortars are generally employed in bonding insulating systems which are exposed to cyclic acidic environments. For example, in an industrial smoke stack, as in a coal-burning power plant, a brick lining has been employed in which the brick lining was bonded with an acid-resistant silicate containing mortar. The mortar and brick were acid resistant in order to avoid damage due principally to the presence of sulfuric acid formed by hydrolysis of sulfur oxides in the stack gases.
The early acid-resistant mortars comprised wet mixtures of silica sand and sodium silicate which were "cured" by evaporation and exposure to acid, either as a wash or while in service. Later, setting agents were developed which were added to the silicate powders. Commonly used silica sol setting agents are sodium silicate glass (ca 3.2 ratio), sodium silicofluoride (Na.sub.2 Si F.sub.6), aluminum phosphate (Al.sub.3 (PO.sub.3).sub.3) and the like. However, utilizing setting agents in silica sol based mortars introduces a foreign and costly compound. Further, the presence of the setting agent may render the silica sol based mortar unsuitable for high temperature service with cellular glass insulation. In addition, the advantage achieved by the use of the silica sol setting agent of rapidly developing mortar strength to permit continuous laying of brick is offset by a substantial reduction in the pot life of the mortar. It is known that trace amounts of moisture in packages of silica sol based mortars containing a sodium silicate glass setting agent initiates first a dissolving stage and ultimately a drop in pot life from about one half hour to about ten minutes in a matter of a few months as a consequence of the uncontrolled hydration of sodium silicate.
Not only are silica sol base mortars containing setting agents costly, the setting agent raises the pH to the range within which gelation occurs within unreasonable times. Nevertheless, the foregoing mortars were developed to bond dense acid resistant brick systems. Brick densities were in the order of 140 pounds per cubic foot which insured sufficient pressure on the mortar joint to promote a good bond. Moreover, the linear coefficients of thermal expansion of brick and prior art mortars were comparable (ca. 7-9 .times. 10.sup.-6 in/in/.degree. F.) so as to minimize spalling and thermal shock failures.
More recently, however, superior insulation compositions have been developed, particularly in the area of cellular glass type compositions. For example, cellular borosilicate glass compositions in the form of blocks or slabs have been developed which are acid resistant and which have excellent insulating properties. The cellular silicates, however, differ substantially from bricks in both physical and thermal characteristics. The cellular borosilicate glasses have a density of only about 9 - 15 pounds per cubic foot and while being an important advantage from one standpoint, that density does not provide sufficient joint pressure to provide a good bond with prior art mortars. A cellular borosilicate glass composition may typically have a linear coefficient of thermal expansion of about 1.7 .times. 10.sup.-6 in/in/.degree. F., far lower than that of bricks or prior art mortars.
As stated above, it has been the conventional practice to control the gelation or setting of silica sol based mortars by the addition of small amounts of a setting agent to the mortar composition, as disclosed in U.S. Pat. Nos. 2,995,453 and 3,024,125. In addition, the particle size distribution of the glass powder, i.e. the specific surface of the fines component, and the relative proportions of the fines to liquid sol must also be controlled to achieve optimum gelation and hardening within a reasonable time. Moreover, it is highly desirable to minimize the sol demand in order to minimize the mortar cost in which the sol component constitutes the most expensive element of the mortar composition and to also lessen mud cracking. It follows that a minimum sol content provides a minimum of fines; however, too few fines gives a sol based mortar that is dry and difficult to work. The addition of an excess of sol, however, leads to "flooding" or "weeping"; while an excess of fines requires an unnecessary additional expenditure of sol which then yields a mortar prone to mud cracking.
Accordingly, there is need for acid-resistant compositions useful in bonding cellular glass bodies in which the composition yields a minimum of sol demand required to yield a trowelable mortar. In addition, there is need for a silica sol based mortar having a composition comparable to that of the cellular glass bodies being bonded and capable of reacting with selected silica sols to yield a gel and subsequently harden into a durable, cementitious mass having good thermal shock characteristics and serviceable under elevated temperatures. Additionally, there is need for a silica sol based mortar composition in which a minimum sol content is provided in the desired ratio with the fines to yield a trowelable mortar that hardens within a reasonable time. Further, there is need for an acid-resistant mortar composition that excludes setting agents and has a constant working life in comparison with known silica sol mortars containing setting agents.